feat: add lambda specifc dsm context extraction code

[michael-zhao459]

What does this PR do?

This PR adds a get_dsm_context_from_lambda() function, which allows for the lamba layer repo to hold the context extraction code instead of the tracer (context extraction function is lambda specific)

Motivation

Annoying to manage the context extraction from both the tracer and lambda layer code

Testing Guidelines

Wrote comprehensive unit tests for the function

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[michael-zhao459]

• feat: add lambda -> kinesis support for dsm #611
• feat: add lambda specifc dsm context extraction code #610 👈 (View in Graphite)
• michael.zhao/dsm-lambda-sns

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